The matter with inner structure in which fine particles of a uniform particle size (particle diameter: 50 nm to 1000 nm, which is called a colloidal particle) are arranged in a three-dimensional periodic manner is called a colloidal crystal. The colloidal crystal appears with coloration by the effect of Bragg reflection of light due to the periodic structure such that it is expected to become a material expressing coloration (structural color) caused by the structure (i.e., material of structural color) wherein the coloration by structure is different from coloration by dye. And a colloidal crystal where resin is filled in space between fine particles is also known (it is also called colloidal crystals immobilized in resin).
In such colloidal crystals, there is a colloidal crystal of non-oriented polycrystals in which crystal domains of from a micron size to a millimeter size in which particles are arranged in a three dimensional periodic arrangement are put together with a randomly orientated relationship with each other (for example, Non-patent reference 1). In such a colloidal crystal of non-oriented polycrystals, condition of angles which gives rise to Bragg reflection differs in each crystal domain such that, as a whole, the structural coloration effect may be obtained in an arbitrary direction. This crystal domain is generally defined as an area constituted of a single crystal.
In another colloidal crystal, there may be obtained a state that crystal domains having sizes not exceeding several millimeters are oriented such that specific crystal lattice planes are orderly arranged in a specific space orientation in the crystal domains. As a manufacturing method of such a colloidal crystal, for example, there is a method to obtain a colloidal crystal having a surface parallel to a base plane by evaporating dispersion liquid gently from a predetermined colloidal solution after spreading the colloidal solution on a flat base plate (for example, Non-patent reference 2). In this case, the colloidal crystal in which a specific crystal lattice plane is oriented in parallel to a surface of a base plate (therefore, the specific crystal lattice plane is also parallel to the surface of the colloidal crystal) may be obtained.
Or, as another method of manufacturing a colloidal crystal, it is also named to produce a colloidal crystal by oscillating relatively parallel planes opposing each other after inserting a predetermined colloidal solution in a narrow gap between the parallel planes wherein the amplitude of the oscillation is set comparable to the gap of the two planes (for example, Patent reference 1). In this case, the whole colloidal crystal may have an orientation state such that the whole colloidal crystal can be substantially regarded as a single crystal since all crystal domains are oriented as the three-dimensional crystal orientation is aligned according to Patent reference 1.
There is a colloidal crystal sheet in which colloidal crystals oriented in such a way are immobilized by elastic member such as polydimethyl silicone (hereinafter, referred to as “oriented colloidal crystal sheet”) (for example, Non-patent reference 2). In the oriented colloidal crystal sheet, since the angle condition to cause the Bragg reflection by a specific crystal lattice plane is to be the same as that in any of the crystal domains, a strong effect of the structural coloration can be obtained in the specific direction.
However, the colloidal crystal of non-oriented polycrystals has a disadvantage that the intensity of coloration is weak since the number of crystal domains that contribute to the Bragg reflection to an arbitrary direction is less than the number of crystal domains that contribute to the Bragg reflection to a specific space orientation in the oriented colloidal crystal sheet (for example, a colloidal crystal sheet in Non-patent reference 2). Here, it is assumed that the size of each crystal domain is the same or comparable.
Further, in a colloidal crystal of Patent reference 1 and an oriented colloidal crystal sheet of Non-patent reference 2, since the crystal domains are oriented such that the specific crystal lattice plane may be arranged to be parallel to the surface of the sheet, if the effect of the structural coloration caused by the Bragg reflection is supposed to be given to an observer who intends to make an observation from the squarely facing direction (i.e., a direction perpendicular to the surface) against the surface of the colloidal crystal and the colloidal crystal sheet, it is necessary to irradiate the surface from the squarely facing direction with the illumination light such that the illumination axis and the observation axis overlap. The above-mentioned configuration is not so practical that it is desired to improve the colloidal crystal.
Therefore, there is a demand to develop a colloidal crystal to achieve the structural coloration effect strongly in the squarely facing direction against the surface of the colloidal crystal wherein the squarely facing direction is the observation direction most naturally selected.